The Lewis negative (Le a--b--) red blood cell phenotype was observed three times more frequently in 170 diabetics (29%) irrespective of their clinical type and in 27 non-diabetics low insulin responders to glucose than in 100 controls (10%). This difference could not be accounted for by factors influencing the serological typing ("ABH secretion and ABO groups) nor by the geographic origin of the populations tested. The Lewis substances are primarly soluble antigens present in blood, saliva, others fluids and absorbed on red blood cells. In 50 diabetics saliva was also analysed. Blood cell and saliva results were concordant allowing to interpret the Lewis negative blood cell phenotype as reflecting the absence of Lewis antigen. The higher frequency of Lewis negative phenotype was not related to the severity or the duration of the diabetes and therefore was unlikely to depend on metabolic factors. The similarity between the results for juvenile and maturity onset diabetes seems to indicate that these two clinical types of diabetes are genetically related. Furthermore, the same results obtained in low insulin responders afford additional support for considering these subjects as potential diabetics. It probably indicates, in the diabetic population, an increased frequency of le/le genotype or of one or several genes inhibiting the expression of Le.